STUDY OF DILUTE FeSn ALLOY

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STUDY OF DILUTE FeSn ALLOY

K. Maring, F. van der Woude, K. Heyman, A. Schaafsma, G. Sawatzky

To cite this version:

K. Maring, F. van der Woude, K. Heyman, A. Schaafsma, G. Sawatzky. STUDY OF DILUTE FeSn ALLOY. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-275-C6-278.

�10.1051/jphyscol:1974641�. �jpa-00215798�

CHARGE AND SPIN DENSITIES IN METALS AND ALLOYS.

STUDY OF DILUTE FeSn ALLOY

K. W. MARING, F. VAN DER WOUDE, K. M. F. HEYMAN, A. S. SCHAAFSMA Solid State Physics Laboratory, Materials Science Center

University of Groningen, Groningen, The Netherlands and G. A. SAWATZKY

Laboratory 'for Physical Chemistry, Materials Science Center University of Groningen, Groningen, The Netherlands

RhumB. - Les resultats des experiences Mossbauer dependant de la temperature sur Fes7 dans le syst6me Fe-4 at % Sn sont prksentes et compares avec des expbiences similaires sur les alliages de FeAl et ESi. Des mesures de magnktisation de masse montrent que A1 et Si se tiennent comme - des dilutants magnktiques simples 2 l'opposk de Sn dissous dans du fer. Les explications possibles pour cette conduite sont discutks, et dans ce contexte les dates prkliminaires de spectroscopie Clec- tronique sont present&..

Abstract. -The results of temperature dependent Mossbauer experiments on Fe57 in the Fe-4 at % Sn system are presented and compared with similar experiments on the FeAl and FeSi alloys. Bulk magnetization measurements show that A1 and Si behave as simple magnetic dilutents in contrast with Sn when dissolved in iron. Possible explanations for this behavior are discussed, and in this context preliminary electron spectroscopy data are presented.

1. Introduction. - A considerable amount of work has been done on dilute alloys of non magnetic impu- rities in a ferromagnetic matrix, by means of neutron diffraction, Mossbauer effect, resistivity and specific heat measurements. Impurities like Al, Si, Ga, Ge, As, Sn and Sb are expected to be non magnetic in Fe because of their electronic structure and indeed neu- tron diffraction measurements [I] have shown that these impurities have no well localized moment in iron at least within an experimental error of about 0.5 pB.

FeAl and FeSi alloys have also been studied by temperature dependent Mossbauer effect in connection with a study of magnetic impurities (3d transition elements) in iron [2, 3,4, 51. These results show that Al and also Si behave as simple dilutents, in agreement with the change of - 2.2 pB per guest atom A1 or Si as determined from bulk magnetization measure- ments [6]. For Sn and also for Sb impurities this change is about - 1 pB per atom [7], indicating no simple magnetic dilution. The corresponding value for Ga, Ge and As impurities lies between these two values namely

- 1 . 4 ~ ~ per guest atom [7, 81.

The results of the studies of non magnetic impurities in ferromagnetic metals are generally explained with the concept of electron impurity screening or a possible s-d admixture. For the study of the FeSn system we there- fore used not only the temperature dependent Moss-

bauer effect but also photoelectron spectroscopy (ESCA), giving a more direct determination of the relative positions of levels and bands.

2. Experimental methods. - The alloys were pre- pared by melting together the pure elements (99.999 %)

in the right composition in a high frequency furnace and an arc furnace, under hydrogen and argon atmosphere respectively. The alloys were checked with X-ray diffraction and chemically analyzed. Because the alloys were too brittle to be rolled, they were powdered by filing. By sieving a maximum grain size of 20 p was obtained. The samples were quenched under hydro- gen atmosphere in water from 800 OC to room tem- perature.

The spectra were recorded with a springless Moss- bauer spectrometer and a 25 mCi source of Co57 in Cr matrix (Fig. 1). For temperatures below room tempe- rature we used a liquid nitrogen cryostat. Above room temperature a furnace with electronic temperature control was used, which kept the temperature constant within 0.2 K. The Curie temperature was obtained by thermal scanning.

For the ESCA experiments we used a slice of 0.3 mm, which was also quenched under hydrogen atmosphere, mechanically polished and argon ion etched in situo.

The ESCA spectra were obtained using an A. E. I.

E. S. 200 spectrometer.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974641

C6-276 K. W. MARING, F. VAN DER WOUDE, K. M. F. HEYMAN, A. S. SCHAAFSMA and G. A. SAWATZKY

( rnrn/s I

FIG. 1. - The outer four absorption lines of a Mossbauer spectrum of Fe-4 at % Sn recorded at room temperature.

3. Results and discussion. - In Fe-4 at % Sn, which has a b. c. c. structure, we can distiguish different iron sites with different surroundings. When the impurity atoms have a random distribution in the alloy, the probabilities of the different iron sites are given by a binomial dist~ibution. In this context H(m, n) will characterize the magnetic hyperfine field at an iron nucleus with m impurities as nearest neighbor and n impurities as next nearest neighbor. The total probabi- lity of the different iron sites used in our analysis was 98 %. We have fitted the Mossbauer spectra with a sum of Lorentzians. The positions, linewidth and depth of the different satellite peaks were obtained by an iterative process. The only constraints were the relative depths, given by the binomial distribution, and the linewidth corresponding with the same nuclear transition, which have been chosen the same. Though several authors assume an additive effect for the impurity atoms, this has not been a constraint in our analysis.

Figure 2 shows the difference between the reduced hyperfine fields. This is in agreement with the results of Vincze et al. [9] but also almost identical with the results of FeAl and - FeSi (Fig. 2).

FIG. 2. - The difference of the reduced hyperfine fields at 57Fe

h((m, 4, T) ⁼ H((0, 01, T ) T

H((m9 n), T ) versus - for FeAl

H((0, ^01, 0) ^- H((m, 4, 0) Tc - (Schurer et al. [3]) and 5%.

Schurer et al. performed [2, 3, 4, 51 temperature dependent Mossbauer effect measurements on FeAl and FeSi and compared these results with those foriron alloyrwith magnetic impurities. By doing this the degree of localization of magnetic moments and exchange in iron and iron based alloys could be determined. The change in the shape of the rnagnetiza- tion curves at Fe57 nuclei in various surroundings, which is characterized by the

h((m, n), T) =

- - H((0, O), T ) - - H((m, n), T ) vs

('1

^.

H((O,O), 0) H((m, n), 0)

could be related to the change in Curie tempera- ture T, of the alloy. In general (except FeCr) positive and negative h corresponded with respectively a nega- tive and positive sign of dT,/dc. The results show that Al and also Si behave as simple dilutents, in agreement with the change of - 2.2 ^p, per guest atom A1 or Si as determined from bulk magnetiza- tion measurements [6]. For Sn impurities the change is - 1 ^p, per atom [7], which indicates no simple magnetic dilution.

We have pe~formed Mossbauer effect measurements of Fe-4 at % Sn between liquid nitrogen temperature andthe Curie temperature. The H,,,(T) curves for Fe5' nuclei in various surroundings in FeSn are almost identical with those obtained for

ex.

According to Stoelinga et al. [lo] also the change in Curie temperature in FeSn and FeAI, with less than 1 at % impurities, are the same namely

dT,/dc = - 0.9 K/at % .

By means of a thermal scanning procedure we find a Curie temperature in Fe-4 at % Sn which is 15.2 K less than for pure iron. Assuming a linear relationship this

STUDY OF DILUTE &Sn ALLOY C6-277

would result in dT,/dc ⁼ - 3.8 K/at %. This indi- cates that the concentration dependence of the Curie temperature in FeSn alloys is not linear. Combining the Mossbauer effect results and the bulk magnetizations the formation of a magnetic moment at the Sn site should not be excluded.

In explaining the properties of both FeX and XFe alloys, with X 3d, 4d and 5d transition~lements>he Mossbauer effect has been very helpful. When AI.S (1, 0) of iron in FeX is plotted versus AI. S. of iron in XFe the transition elements lie on a straight line,

-

the non-transition elements Al, Si, Ga, Ge, Sn and Sb also lie on a straight line however the slope has an opposite sign with respect to the transition ele- ments [I 11.

For transition elements the increase in isomer shift, when going from left to right in the periodic table, is explained by transfer of d electrons to the central iron atom and of s electrons from the central iron atom, meeting the neutrality principle. From an analysis of hyperfine field and isomer shift data by Schurer et al. [12] results that both the number of 4s and 3d electrons decreases on going from the left to the right in the periodic table, suggesting that the charge neutra- lity model suggested by Ingalls [13] is not applicable.

Because of the different electronic structure of non- transition elements with respect to iron it is very diffi- cult to predict which levels mix or form localized states.

Covalent bonding may play a role in alloys of iron and these non-transition elements.

It is interesting to note that there is a close relation between the hypelfine field and the isomer shift. For example, when we plot AH(1,O) vs AI. S (1,O) for FeX alloys, with X transition elements, a straight lineis found [12]. However, doing so for the non-transition elements Al, Si, Ga, Ge, As, Sn and Sb, using the values of Vincze et al. we also get straight lines, but with different slopes for the different columns in the periodic table (Fig. 3).

From bulk magnetization measurements on alloys of iron with non-transition elements dpldc is found to be dependent on the rows of the periodic table 191.

0.03 0.06 0.09

Ail ( m m / s )

-

FIG. 3. - The change in hyperfine field and isomer shift at an iron site with one impurity in the first shell (from

data of Vincze et al. [9]).

The phenomenological model of Schurer et al. [2, 3, 4, 51 can be used to investigate the existence of local moments and the range of these moments but it is very difficult to find out something about the electronic structure at and around an impurity, especially while not only s and d electrons are involved but also p electrons. The Mossbauer eEect alone only gives two parameters in our case, i. e. the magnetic hyperfine field and the isomer shift. In that case one has to make assumptions e. g. the neutrality condition. Additional information is obtained by electron spectroscopy which gives more direct information about the electronic structure.

In an attempt to explain the systematics in the bulk magnetization measurements Vincze et al. 191 assume the 3s' levels in A1 and Si, the 4s2 levels in Ga, Ge and As and the 5s' levels in Sn and Sb to be well localized and lying below the iron 3d states. The change in iron moment is explained by interaction of the iron 3d states and the corresponding 3s2, 4s' and 5s2 levels.

This argument is only valid when the corresponding s levels in a row of the periodic table have the same energy difference with respect to iron 3d states, which is not expected considering the binding energies in the elements.

Another possibility to explain the bulk magnetiza- tion measurements is the assumption of a magnetic moment on the Sn atom in &Sn alloys, which is suggested by Huffman et al. [14] in the case of CoSn alloys. In order to investigate this possibility as well as screening effects, we performed electron spectroscopy (ESCA) experiments on Fe, Sn and Fe-4 at % Sn. The ESCA technique is a very direct way to determine the relative positions of electron levels and bands, SO

charge and spin effects upon alloying may be visible when we compare the ESCA results of the alloy and the pure elements.

Screening effects might cause a shift of for instance the p levels and the presence of a moment on Sn atoms might cause multiplet splitting. Because of the small Sn content in Fe-4 at % Sn, the contribution of inelasti- cally scattered electrons and possible oxygen contami- nation the resolution is yet rather poor, but pieliminary results show no significant shift (at most 0.1 eV) of the Sn levels in FeSn compared with pure Sn, indicating that screeningeffects take place within one unit cell.

In pure Sn the width of the 3d and 4d levels is rather small (z 1.3 eV) in contrast with the 3s, 3p, 4s and 4p levels which are very broad ( z 5 eV). In FeSn the Sn 3d level is broadened a little ( z 0.1 eV) and shows shoulders, the 3s, 3p, 4s, 4p and 4d levels are again very broad, but the statistics were insufficient to make reliable conclusions.

The experiments will be continued with higher Sn concentrations and with other non-transition element impurities. So far, we can conclude that charge neutra- lity holds in FeSn alloy and that the question of the formation o f a magnetic moment at Sn sites is still open. A complete Mossbauer analysis of H versus T

C6-278 K. W. MARING, F. VAN DER WOUDE, K. M. F. HEYMAN, A. S. SCHAAFSMA and G. A. SAWATZKY

curves at 57Fe and "9Sn nuclei in these alloys will be Fundamenteel Onderzoek der Materie (F. 0. M . ) and postponed till this question has been settled. Stichting voor Scheikundig Onderzoek Nederland (S. 0. N . ) with financial support from the Nederlandse Acknowledgments. - This work was performed as Organisatie voor Zuiver Wetenschappelijk Onderzoek a part of the research program of the Stichting voor (Z. W. 0.).

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